While NW can route the bags BWI-MSP-SMF-MSP-DTW, the problem is that in SMF, the baggage handlers are not going to be looking for transfer bags, especially if the transfer is the same aircraft.

If I were you I would either ship the bags or schedule a later flight from SMF (or earlier arrival). At check-in, they might agree to check them BWI-MSP-DTW or BWI-DTW but if they don't, it's too late to ship.

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"Don't be sorry. Be quiet!" -- Minister, Spaceballs